Why does a word in English have many meanings, and the meanings are very different?

Personally, I think that "polysemy" is an inevitable phenomenon in any language, which is caused by "different usage", but "different etymology" only exists in synonyms, because many synonyms are often distinguished by different application fields or objects. Polysemy is very common in English, mainly because English speakers are often accustomed to "borrowing rather than using the original meaning", which is also the main reason for the proliferation of "metaphor" usage in English.

Theoretically speaking, each word has only one original meaning, and other meanings are derived meanings or derived meanings developed from the original meaning. The "meaning is very different" mentioned in your formulation is actually a misunderstanding: the meaning of polysemous words is actually "extremely related", and other meanings are derived from the original meaning. Most of the literal differences are often caused by metaphors, personification, metonymy, abstraction and other usages (rhetorical devices), and only a few words will escape (meaning changes) in the process of development.

Therefore, the ambiguity of most English words can be basically solved from two angles: word formation (how words are made) and word use (how the meaning of words is used), and in some cases, it is necessary to refer to or analyze their etymology (see their ancestors' eighteen generations). In this way, many meanings of polysemous words can be connected in series through their original meanings, which is the fundamental way to remember words for a long time. Otherwise, even if you read and recite words every day, it will be "temporary solution, not permanent cure".

Personal opinion, for reference only.